1. The Field of the Invention
This invention relates to coated sheet steel and more particularly to an improved sheet steel having a thin coating of a nickel-zinc alloy electrodeposited on its surfaces. The invention also relates to an improved drawn and ironed can formed from sheet steel having a nickel-zinc alloy coating plated thereon.
2. Description of the Prior Art
Plated sheet steel is well known and widely used for various applications particularly where corrosion resistance is an important consideration. In the past, tin has been the most common coating metal and tinplate has been widely used particularly in the production of cans for food, beverages, and the like. The use of a chromium plated steel is also well known in the production of cans, and galvanized (zinc coated) steel and nickel plated steel has also been used for various purposes.
It is also well known to form containers, or cans, and particularly beverage cans, by the draw and iron process. This procedure involves drawing an initially flat metal blank into a cup having side and bottom wall thicknesses which are substantially equal, with the height of the cup being substantially less than that of the final can and the diameter being substantially equal to that of the finished can. The drawn cup is then supported on a punch or mandrel and forced through one or more ironing dies or rings whose inside diameter are each smaller than the outside diameter of the cup passing therethrough so that pressure between the ironing dies and the mandrel progressively reduces the thickness of the sidewall of the drawn cup and forces the metal along the mandrel to increase the height of the can.
In the past, difficulty has been encountered in forming drawn and ironed can bodies from uncoated flat rolled steel. The substantial forces required frequently resulted in tearing of the thinned sidewall, or pushing the mandrel through the can bottom. Thus, it has generally been found necessary to provide a coating of a softer metal such as tin on both sides of the steel base metal as a lubricating coating in order to successfully iron the sidewalls to the desired thickness.
It has generally been considered necessary to provide a coating of at leasat 0.25 pounds of tin per base box of steel (quarter pound plate) in order to assure reliable solderability of the components in a three-piece tin plate can. Coating weights at least as great as for three-piece containers have generally been found necessary to draw and iron tin plate containers, and half pound plate (0.50 pound tin per base box of steel) has been used to produce drawn and ironed tin-plated cans on a high-speed commercial production line. Half pound tin plate has a coating thickness of approximately 30 microinches on each side. This heavy tin coating greatly increases the cost of completed cans formed from tin plate. Further, in forming such drawn and ironed cans, there is a tendency for the tin to flow or be drawn from plateaus and deposited in valleys of the base steel surface, particularly during the ironig step, with the result that the thickness of the tin coating on the finished product varies widely.
Substantial effort has been made in recent years to provide a commercially acceptable alternative to the tin can, i.e., a can formed from tin plate. For example, chrome-chrome oxide coatings are widely used in the production of three-piece cans although it generally has not been considered possible to form a drawn and ironed container from such chrome-plated steel. U.S. Pat. Nos. 3,245,885 and 3,295,936 disclose the use of steel plate having a thin nickel coating, with the nickel coating being electrochemically treated in a solution of dichromate or chromic acid to deposit a thin film on the nickel to improve corrosion resistance and lacquerability. These patents also suggest substituting a nickel-iron or nickel-tin alloy for the thin nickel coating. The nickel coating is stated to be about 0.02 to 0.3 microns (about 0.8 to 11.8 microinches) in thickness, and the coated and treated steel material is stated to have anticorrosiveness, lacquerability and solderability about equal to that of tin plate. The '885 patent states that the coating is so thin that the workability is high.
U.S. Pat. No. 3,978,803 discloses a sheet steel material having a first layer of nickel or copper plated on its surfaces, with an outer layer of tin plated over the nickel or copper layer. The doubly-coated sheet steel is asserted to be useful in the forming of drawn and ironed cans.
It is common for small amounts of various metals to appear as impurities in continuous electroplating baths. For example, trace amounts of zinc are frequently present as an impurity in nickel plating solution. An article by D. T. Exing et al. appearing in the September, 1952 issue of Plating (Plating, Vol. 39, No.9, page 1033) reports the results of testing conducted at Michigan State College to determine the effects of zinc as an impurity in a nickel plating bath, and discusses procedures for removing the impurity from the bath. According to this article, the presence of zinc increases the hardness and adversely affects ductility of the nickel coating.
It generally has not been considered practical to form drawn and ironed containers from thin nickel coated steel on a high-speed commercial production line. While cans can be drawn and ironed from such material under laboratory conditions, experimentation has shown that numerous variables are so critical that they cannot always be reliably controlled to the extent necessary for successful high-speed commercial drawing and ironing line operation.